Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna and an RFID integrated circuit (IC) including a radio section, a power management section, and frequently a logical section and a memory. In some RFID tags the power management section employs an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
At least a portion of the IC memory is typically implemented as nonvolatile memory (NVM). An NVM comprises one or more memory cells, whose contents may be changed by a write operation. If the NVM employs floating-gate memory cells then the write operation often uses electron tunneling, where a high voltage applied across an oxide surrounding a floating gate induces electrons to tunnel onto or off of the floating gate. Because the physical characteristics of the memory cells may vary due to manufacturing tolerances, oxide thicknesses, etc., electron tunneling often employs a succession of voltage pulses of increasing amplitude, each of which is followed by a data-verification step. The pulses are stopped when the memory cell contains the proper value. This iterative write-verify process writes data to the NVM without prior knowledge of the required tunneling voltage and, at the same time, prevents over-tunneling because each new tunneling pulse ramps to only a slightly higher voltage then the prior pulse. Unfortunately, this approach typically wastes a substantial amount of time by slowly ramping the tunneling voltage from a safe, low value to the required value, where this required value may not be appreciably different from the first time that the memory cell was written.